Researchers found that high testosterone levels in men caused a
lower immune response to vaccination against influenza, making them
more vulnerable to the illness. Women and men with low testosterone both had a more active immune response to vaccination than men with high levels of testosterone.

"This is the first study to correlate poor immune response to testosterone levels in men," says Mark Davis, PhD,
the study's senior researcher and a professor of microbiology and
immunology at Stanford School of Medicine.

The findings may help explain
why men are more susceptible to viral and bacterial infections than
women.

High Testosterone and Men's Increased Susceptibility to Infections
The immune system defends the body against foreign invaders like
viruses and bacteria. The success of a flu vaccination depends on your
immune system's ability to make antibodies against flu virus.

In the PNAS study, the researchers looked at proteins found in the
blood of 53 women and 34 men given a flu shot. These proteins, called
immune-signaling proteins, told the researchers how strong the response
was.

High testosterone levels correlated with lower response.

The researchers also found a gene called Module 52 that was highly
activated in men with higher levels of testosterone. "This gene seems to
be turned on by testosterone," Dr. Davis says.

"Once it gets turned on,
it decreases the immune response. These findings suggest a genetic
mechanism for the sex differences observed in the human immune system."

It's already known that sex hormones play a role in
immune response. The female hormone estrogen increases immune system
inflammation, and the male hormone testosterone decreases it.

Men are
more likely to get infections, and women have a better response to
vaccines.

But these sex differences in health aren't black and white. Women can
have an overactive immune system.

When the immune system becomes
overactive, it can cause a type of disease called autoimmune disease.

Examples of autoimmune diseases include multiple sclerosis, rheumatoid
arthritis, and lupus -- all diseases that are much more common in women.

Evolution and the Immune System
Finding a genetic link between high testosterone and low immune
response raises the question of why nature would design the immune
system to be influenced by hormone levels.

"Pregnancy may be part of the reason," says Davis. "We know that a
woman's immune system needs to be turned up after pregnancy to protect
her and her baby."

A woman's immune system gets revved up by estrogen,
and this hyper-vigilance may be what causes more frequent autoimmune disease in women.

Evolution may play a role, too, says Natan Bar-Chama, MD,
director of male reproductive medicine and surgery at Mount Sinai
Medical Center in New York City.

"There could also be an evolutionary
advantage for men to have a weaker immune response," he says. "A weaker
immune response may have helped men survive trauma and injury,
increasing their chances of survival."

Davis seems to agree. "During evolution, men would be more likely to
clash violently with each other," he says.

"A weaker response may be a
disadvantage when it comes to infections, but an advantage when it comes
to a severe immune response.

We know that women are less likely to
survive a severe immune response, called a cytokine storm."

What This Means for Men's Health
The study also raises the issue of flu vaccine effectiveness. "This
was a sophisticated study that sheds light on the role of testosterone
in the immune response of humans, but it is still a small study," says
Dr. Bar-Chama.

"It does not mean that men should not get flu shots. Less
effective does not mean not effective."

The immune system is complicated and relies on a delicate balance to
function well. "It may be that men benefit from a somewhat weakened, but
not too weakened immune response," Davis says.

"But men who take testosterone
to boost their testosterone levels should take note." And, though the
idea of boosting the immune system may seem exciting, the ability to
actually do it has proved to be elusive.

On the other hand, "we know that low testosterone
is linked to increased mortality in men due to bone health and
cardiovascular health," Bar-Chama says.

"So men who have low
testosterone along with symptoms of low testosterone can benefit from
testosterone restoration therapy."

How to Boost Immune System Health
"I am not aware of any vitamins or supplements that have been shown to boost the immune system,"
says Bar-Chama, "Good health is your best booster. That includes
reducing stress and getting enough sleep." Other smart lifestyle
suggestions: